OMC Cobra Outdrive

" I need Help.. Seems no one l

" I need Help.. Seems no one local can grasp my question!

I have a 1987 Bayliner Trophy 24.5 foot, about 3800lbs. It came with a 305 sbc OMC Cobra sterndrive. in it's prime the 305 turned 4800 rpm and produced 29 mph. For numerous reasons I purchased a complete power unit from a 1988 Bayliner Sunbridge This boat weighed about 8000 pounds it did 25 mph with this power unit. It is a 350 sbc with OMC drive and so forth.

I swapped the 350 in and the 305 out, went like butter. However, the engine turned about 4200 rpm (which is fine) but only pushed 22-23 mph. This was not the goal! I then swapped drives using the new drive i bought from the bigger boat. I used the same 15x17 prop on all tests. The new engine with the new drive now only turns at 3200 rpm wot but still produces 22-23 mph. GO figure.

I can only keep one drive.

My Question is: should I keep the new drive that only allows 3200 rpm wot and play with the prop size/pitch to try to increase rpm and speed?? or should I put the original drive back and get another prop to try to increase speed? By the way the gas mileage is Horrible and the boat only does a semi plane kinda thing..

CAN ANYONE HELP?? or even follow this??

Thanks "

" The 305 and 350's have d

" The 305 and 350's have different speed/torque curves. The 305 turned out the highest HP at a higher RPM than the 350.

Additionally, the Sundancer was (probably) a dual-motor system. The power from the second motor helped both motors get up-to-RPM. so that's why you're seeing the problem.

You're going to have to change props in order to get optimized performance from the 350. The prop that came from the Sunbridge was optimized for an 8000 lb dual-motor boat, and your original prop was optimized for the 305's speed-torque curve. It goes without saying that you should be as close as possible to your HP peak at Wide-open-throttle. So it looks like you're going to have "too much prop" using the Sunbridge's prop (you don't have the second motor to get your RPM's up) and you'll have "too little prop" with the old 305's.

Boat designers have expensive software designed to optimize props for system installations. You don't have it, so you'll need to find a source that will let you try-before-you-buy. Or check with a boatyard, they might have a suggestion. Marinas are useless for this.

Sorry I can't tell you which prop to use. If you contact Bayliner, they may be willing to give you the info, because they may have had a 350 option for your boat.

Good luck

Jim "